Rotary electric machine

ABSTRACT

A rotary electric machine is composed of an armature core, an armature winding, a rotor core disposed opposite said armature core, a rotary magnetic-flux source for supplying first magnetic flux to the rotor core, a frame for supporting the armature core and the rotor core and a stationary magnetic flux source, fixed to the frame, for supplying second magnetic flux to the rotor core in a direction to supplement the first magnetic flux.

CROSS REFERENCE TO RELATED APPLICATION

[0001] The present application is based on and claims priority from thefollowing Japanese Patent Applications: 2002-77774, filed Mar. 20, 2002and 2002-117775 filed Apr. 19, 2002, the contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a compact and powerful rotaryelectric machine to be used for a passenger car, an airplane and anelectric power tool.

[0004] 2. Description of the Related Art

[0005] A typical rotary electric machine to be used for a vehicle is analternator that has a Lundell type rotary magnetic core. The Lundelltype rotary magnetic core is composed of a boss portion, a pair of diskportions disposed at opposite ends of the boss portion and a pluralityof claw poles extending from the disk portions in the axial direction ofthe core to alternately interleave with each other. A cylindrical fieldcoil is wound around the boss portion to supply magnetic flux toparallel magnetic circuits that respectively include the claw poles.Therefore, comparatively large magnetomotive force can be applied to theclaw poles, so that the alternator can generate comparatively highpower.

[0006] However, the thickness of the claw poles have to be limited inorder to limit the outside diameter of the alternator. This limitationmay cause DC magnetic saturation, which limits an amount of effectivemagnetic flux and output power of the alternator.

[0007] In order to increase the effective magnetic flux, it has beenproposed that a permanent magnet is inserted between adjacent clawpoles. However, the space for accommodating the permanent magnets islimited and cooling performance of the alternator may get worse becausethe permanent magnets close air passages.

SUMMARY OF THE INVENTION

[0008] Therefore, a main object of the invention is to provide a morecompact and powerful rotary electric machine.

[0009] According to a feature of the invention, a rotary electricmachine includes an armature core and an armature winding mounted in thearmature core, a rotor having a rotor core, a rotary magnetic-fluxsource fixed to the rotor core, a frame and a stationary magnetic fluxsource fixed to the frame. The stationary magnetic flux is supplied tothe rotor core in a direction to supplement the first magnetic flux.Therefore, effective magnetic flux supplied to the armature core can beincreased without increasing the size of the rotor. Because thestationary magnetic source can be disposed in a dead space of the frontframe, the frame does not increase the size. The second magnetic fluxsource may include a yoke for magnetically connecting the armature coreand the rotor core and a stationary field coil for providing dcmagnetomotive force in a direction opposite the polarity of the rotorcore. The rotary or stationary magnetic flux source may be composed of apermanent magnet. In such a case, the other includes a field coil. Thefield current supplied to the field coil is controlled to changemagnetic flux supplied to the armature core.

[0010] Another object of the invention is to provide an electric rotarymachine whose rotor has a reduced moment of inertia in order to rotateas soon as it is started.

[0011] According to another feature of the invention, a rotary electricmachine includes a stator core, a three-phase armature winding and afield coil, an inductor rotor disposed opposite the stator core via afirst air gap and a magnetic circuit means for connecting the rotor, thestator core via a second air gap. The inductor rotor is composed of aplurality of magnetically conductive portions and magneticallynon-conductive portions that are alternately disposed in thecircumferential direction thereof between the first air gap and thesecond air gap.

[0012] Because the inductor does not include a cylindrical field coil orclaw poles, the moment of inertia thereof is very small as compared tothe rotor having a Lundell type pole cores. When the motor-generator isoperated as a motor, the inductor can rotates in a very short time afterarmature current is supplied by the inverter because of the small momentof inertia of the inductor.

[0013] The inductor rotor may include a plurality of permanent magnetshaving the same polarity disposed in the circumferential directionthereof at two magnetic pole-pitches. The field coil may be disposedinside the inductor rotor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Other objects, features and characteristics of the presentinvention as well as the functions of related parts of the presentinvention will become clear from a study of the following detaileddescription, the appended claims and the drawings. In the drawings:

[0015]FIG. 1 is a schematic cross-sectional diagram of a main portion ofan ac generator for a vehicle according to the first embodiment of theinvention;

[0016]FIG. 2 is a graph showing two magnetic flux waves generated in theac generator;

[0017]FIG. 3 is a graph showing a power characteristic of the acgenerator;

[0018]FIG. 4 is a schematic cross-sectional diagram of a main portion ofan ac generator for a vehicle according to the second embodiment of theinvention;

[0019]FIG. 5 is a schematic cross-sectional diagram of a main portion ofan ac generator for a vehicle according to the second embodiment;

[0020]FIG. 6 is a schematic cross-sectional diagram of a main portion ofan ac generator for a vehicle according to the third embodiment of theinvention;

[0021]FIG. 7 is a schematic cross-sectional diagram of a main portion ofan ac generator for a vehicle according to the third embodiment of theinvention;

[0022]FIG. 8 is a cross-sectional side view of an ac generator accordingto the fourth embodiment of the invention;

[0023]FIG. 9 is a cross-sectional plan view of the ac generatoraccording to the fourth embodiment;

[0024]FIG. 10 is a circuit diagram of the ac generator according to thefourth embodiment;

[0025]FIG. 11 is a cross-sectional side view of a motor generatoraccording to the fifth embodiment of the invention;

[0026]FIG. 12 is a cross-sectional plan view of a main portion of themotor generator according to the fifth embodiment;

[0027]FIG. 13 is a circuit diagram of the motor generator according tothe fifth embodiment; and

[0028]FIG. 14 is a cross-sectional side view of an ac generator for avehicle according to the sixth embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] An ac generator for a vehicle according to the first embodimentof the invention will be described with reference to FIGS. 1-3.

[0030] The ac generator 1 is composed of an armature 2, a rotor 3, acylindrical stationary field coil 4, a front frame 5, a rear frame 6, afront bearing 8, and a rear bearing 9 etc.

[0031] The armature 2 is composed of an armature core 21, a three-phasearmature winding 23 that is mounted in a plurality of slots formed inthe armature core 21. The armature winding 23 has output lead wiresconnected to a three-phase full-wave rectifier unit (not shown), whichprovides DC output power.

[0032] The rotor 3 is composed of a cylindrical rotary field coil 31, apair of Lundell type magnetic pole cores 32 each of which has six clawpoles extending to enclose the rotary field coil 31 and a rotary shaft33. Field current is supplied to the rotary field coil 31 via a pair ofbrushes and a pair of slip rings (which are not shown but well known).

[0033] The front frame 5 and the rear frame 6 accommodate the armature 2and the rotor 3 so that the rotor 3 and the rotary shaft 33 can rotateinside the armature 2 and so that the peripheral surfaces of the clawpoles can face the inside surface of the armature core 21 through afirst air gap. The front frame 5 is made of soft iron and has astationary magnetic pole 51 at a portion that faces an axial end of oneof the pole cores 32 through a second air gap. Thus, the front frame 5functions as a magnetic yoke that magnetically connects the armaturecore 21 and the pole cores 32. On the other hand, the rear frame 6 ismade of aluminum or other nonmagnetic material.

[0034] The stationary field coil 4 is disposed inside the front frame 5and fixed to a radially outer surface of the stationary magnetic pole51. The stationary field coil 4 supplies magnetic flux to the armature 2through a magnetic circuit in parallel with a magnetic circuit of therotary field coil 31. When field current is supplied to the rotary fieldcoil 31, the pole core 32 on the side of front frame 5 is polarized tobe magnetic N-pole, and the pole core 32 on the other side is polarizedto be S-pole. Accordingly, alternating magnetic flux A is supplied tothe armature core 21 when the rotor rotates. When field current issupplied to the stationary field coil 4, the stationary pole 51 ispolarized to be N-pole. Accordingly, one way magnetic flux B is suppliedfrom the stationary magnetic pole 51 to the armature core 21 as long asthe field current is supplied to the stationary field coil 4. When fieldcurrent is supplied to both the rotary field coil 31 and the stationaryfield coils 4, the armature core 21 is supplied with magnetic flux A andmagnetic flux B, as shown in FIG. 2. In other words, the magnetic fluxsupplied from the claw poles of the magnetic pole core 32 on the side ofthe front frame 5 is larger than the magnetic flux from the claw poleson the other side.

[0035] Therefore, effective magnetic flux supplied to the armaturewinding 23 is increased by the stationary field coil 4 and thestationary magnetic pole 51, which can be disposed in a dead space ofthe front frame. As a result, the output power can be increased byapproximately 30%, as shown in FIG. 3, without increasing the size ofthe generator.

[0036] In the above embodiment, the stationary field coil 4 can bedisposed inside the rear frame instead of the front frame, or anotherstationary field coil can be added to the rear frame 6. In this case,the rear frame 6 is preferably made of soft iron.

[0037] An ac generator 1A according to the second embodiment of theinvention will be described with reference to FIGS. 4-5.

[0038] The rotary field coil 31 of the ac generator 1 according to thefirst embodiment is replaced by a disk-shaped permanent magnet 131.Therefore, the ac generator 1A is composed of an armature 2, a rotor 3A,a first stationary field coil 4A, a second stationary coil 4B, a frontframe 5A, a rear frame 5A, and yokes 71, 72, 73, 74.

[0039] The armature 2, which is the same in structure as the acgenerator according to the first embodiment, has an armature core 21 andan armature winding 23. The rotor 3A has a pair of magnetic pole cores132 each of which has six claw poles enclosing the disk-shaped permanentmagnet 131 and a rotary shaft 133. The front frame 5A and the rear frame6A are made of aluminum and accommodate the armature 2 and the rotor 3Atherein in the same manner as the first embodiment.

[0040] The yoke 71 is an L-shaped member made of soft iron having oneside fixed to the inner surface of the front frame 5A and the other sideextending in the axial direction of the frame 5A. The yoke 72 is fixedto the yoke 71 so as to hold the first stationary field coil 4A togetherwith the yoke 71. The yoke 72 is disposed opposite the pole core 132 onthe side of the front frame 5A so as to function as a stationary polecore that is polarized to have the same magnetic pole as this pole core132.

[0041] The yoke 73 is an L-shaped member made of soft iron having oneside fixed to the inner surface of the rear frame 5B and the other sideextending in the axial direction. The yoke 74 is fixed to the yoke 73 soas to hold the second stationary field coil 4B together with the yoke73. The yoke 74 is disposed opposite the pole core 132 on the side ofthe rear frame 6A so as to function as a stationary pole core that ispolarized to have the same magnetic pole as this pole core 132.

[0042] A magnetic circuit through which the magnetic flux of the firststationary field coil 4A flows is formed in parallel with a magneticcircuit through which the magnetic flux of the permanent magnet flows.As shown in FIG. 4, the pole core 132 on the side of the front frame 5Ais polarized by the permanent magnet 131 to be N-pole, and the yoke 72is polarized to be N-pole when the first stationary field coil 4A issupplied with field current. On the other hand, the pole core 132 on theside of the rear frame 6A is polarized by the permanent magnet 131 to beS-pole, and the yoke 74 is polarized to be S-pole when the secondstationary field coil 4B is supplied with field current. Thus, theeffective magnetic flux supplied to the armature winding 23 can beincreased.

[0043] When no field current is supplied to the first and secondstationary field coils 4A, 4B, the flux of the permanent magnet 131flows through the pole core 132 on the side of the front frame, the yoke72, the yoke 71, the armature core 21, the yoke 73, the yoke 74 and thepole core 132 on the side of the rear frame. Therefore, no magnetic fluxcross the armature winding. As a result, no power is generated when nocurrent is supplied to the stationary coils although the ac generatorhas a permanent magnet.

[0044] On the other hand the output power can be easily controlled bychanging the field current supplied to the stationary field coils 4A,4B. In this embodiment, the front and rear frames can be made of softiron so that the yokes can be integrated therewith.

[0045] An ac generator 1B according to the third embodiment of theinvention will be described with reference to FIGS. 6 and 7.

[0046] As shown in FIG. 6, the ac generator for a vehicle is composed ofan armature 2, a rotor 3B, a front frame 5B, a rear frame 6B, a firststationary permanent magnet 81 and a second stationary permanent magnet82. The armature 2, which is basically the same as the armature of theac generator according to the first embodiment, is composed of anarmature core 21 and an armature winding 23. The rotor 3B has permanentmagnets 34 between adjacent claw poles in addition to the components ofthe rotor 3 of the ac generator according to the first embodiment. Thefront frame 5B and the rear frame 6B are made of soft iron so as tofunction as a magnetic yoke and accommodate the armature 2 and the rotor3B in the same manner as the previously described ac generators.

[0047] The first stationary permanent magnet 81 is disposed at a portionof the inside wall of the front frame 5B opposite the front surface ofthe pole core 32 that is disposed on the front end of the rotor 3B. Thepermanent magnet 81 is magnetized so that rear surface of the permanentmagnet 81 has the same polarity as the front surface of the said polecore 32. The second stationary permanent magnet 82 is disposed at aportion of the inside wall of the rear frame 6B opposite the rearsurface of the pole core 32 that is disposed on the rear end of therotor 3B. The permanent magnet 82 is magnetized so that front surface ofthe permanent magnet 82 has the same polarity as the rear surface of thesaid pole core 32.

[0048] Thus, the magnetic flux of the permanent magnets 81, 82 can beadded to the magnetic flux of the rotary field coil 31 so that theoutput power of the ac generator can be increased.

[0049] When no field current is supplied to the rotary field coil 31,the composite magnetic flux of the permanent magnets 81, 82 flows fromthe permanent magnet 81 through the pole core 32 on the side of thefront frame, the pole core 32 on the side of the rear frame, thepermanent magnet 82, the rear frame 6B and the front frame 5B to thepermanent magnet 81. In addition, the magnetic flux of the permanentmagnet 34 flows through the pole core on the side of the front frame 32,the pole core on the side of the front frame 32 to the permanent magnet34. Therefore, no magnetic flux cross the armature winding. As a result,no power is generated when no current is supplied to the stationarycoils although the ac generator has a permanent magnet.

[0050] In this embodiment, the front and rear frame can be made ofnonmagnetic material if the portions of the magnetic circuit of theframes are replaced by yokes as shown in FIG. 4.

[0051] An ac generator for a vehicle according to the fourth embodimentof the invention will be described hereafter with reference to FIGS.8-10.

[0052] The ac generator includes a front frame 5 made of cast iron, athree-phase armature winding 23, a cylindrical stator core 21 in whichthe stator winding 23 is mounted, a cylindrical inductor 3C made oflaminated iron sheets disposed inside the stator core 21, a non-magneticretainer plate 35, a rotary shaft 33, a rear frame 6 made ofnon-magnetic material, a front bearing 8, a rear bearing 9, acylindrical field coil 4 and a plurality of permanent magnets 34. Thenon-magnetic retainer plate 35 is disposed at an end of the inductor 3Cto fix the inductor 3C and the rotary shaft 33 together. The front frame5 and the rear frame 6 are coupled together to hold the stator core 21.The inductor 3C and the shaft 33 are rotatably supported by the frontand rear bearings 8, 9. The front frame 5 has a cylindrical core portionthat axially projects into the inside of the inductor 3C. Thecylindrical core portion has an inner bore through which the rotaryshaft 33 extends so as to freely rotate. The cylindrical core portionalso has an end portion having a smaller outside diameter around whichthe inductor 3C is disposed and a base portion having a larger outsidediameter around which the field coil 4 is wound.

[0053] The stator core 21 has six teeth around which the armaturewinding 23 is wound, as shown in FIG. 9. The armature winding 23 hasthree output ends that are connected to a rectifier unit 11 to providedc output power at an output terminal 12, as shown in FIG. 10. Afield-current control unit 13 is connected to the field coil 4.

[0054] The inductor 3C is composed of a outer ring, an inner ring and aplurality of honeycomb shaped slots between the outer and inner rings.Two rings are magnetically connected by a pair of diametrically formedthick spoke members. The outer ring is so thin that no magnetic circuitcan be formed thereby. The inner ring forms a portion of a magneticcircuit. The permanent magnets 34 are disposed at two pole-pitches inthe circumferential direction of the inductor 3C. Thus, the inductor 3Chas magnetically conductive portions and magnetically non-conductiveportions.

[0055] The rotary shaft 33 carries a pulley, which is rotated by anengine via a belt.

[0056] When the rotary shaft 33 is driven by an engine via a pulley, theinductor 3C is rotated by the shaft 33 via the retainer plate 35. Whenfield current is supplied to the field coil 4 by the field currentcontrol unit 13, the field coil 4 generates magnetic flux flowing fromthe field coil 4 through the front frame 5, the stator core 21, theinductor 3C to the cylindrical core portion of the front frame 5. Whenthe inductor 3C rotates, magnetic flux flowing through the inductor 3Cchanges because of the magnetically conductive portions and themagnetically nonconductive portions of the inductor 3C. Accordingly, acvoltage is induced in the armature winding 23. Because the permanentmagnets 34 are disposed at two pole pitches and magnetized to have apolarity opposite the direction of the magnetomotive force of the statorcore 21, the amplitude of change in the magnetic flux that crosses thearmature winding is increased by the permanent magnets. The ac voltageis converted into dc voltage by the rectifier unit 11.

[0057] Thus, the outside diameter of the inductor 3C can be made verysmall as compared by a rotor having a Lundell type pole cores becausethe inductor 3C does not include a cylindrical field coil or claw poles.Because the inductor 3C is made of laminated iron sheets that havehollow slots, the moment of inertia thereof is very small as compared tothe rotor having a Lundell type pole cores.

[0058] For example, the moment of inertia is compared between a rotorthat has a Lundell type pole cores of an ac generator whose rated poweris 100 A and an inductor of an ac generator according to the inventionwhose rated power is approximately the same. The rotor, which has aLundell type pole cores, has about 28 kg-cm², while the inductor hasabout 7 kgm². Thus, the moment of inertia of the inductor 3C is aboutone fourth of that of the rotor having a Lundell type pole cores.

[0059] A motor generator according to the fifth embodiment of theinvention is described with reference to FIGS. 11-13.

[0060] The motor generator includes a cylindrical stator core 21 inwhich a stator winding 23 is mounted, a cylindrical inductor 3C made oflaminated iron sheets disposed inside the stator core 21, a non-magneticretainer plate 35, a cylindrical field coil 4 and a plurality ofpermanent magnets 34, a frame 5, a yoke 71 and a rotary shaft 33. Thenonmagnetic retainer plate 35 is disposed at an end of the inductor 3Cto fix the inductor 3C and the rotary shaft 24 together. The frame 5 andthe yoke 22 hold the stator core 21. The stator core 21 has a pluralityof teeth around which the armature winding 23 is wound. The armaturewinding 23 has three output ends that are connected to an inverter unit17. The armature winding 23 and the field coil 4 are connected in seriesso that starting torque can be increased. A field-current control unit13 is connected to the field coil 4.

[0061] The inductor 3C is composed of an outer ring, an inner ring and aplurality of rectangular slots between the outer and inner rings. Tworings are magnetically connected by a plurality of spoke members 36. Theouter ring is so thin that no magnetic circuit can be formed thereby.The inner ring has a thickness of about ⅙ of the distance between theouter ring and the inner ring and forms a portion of a magnetic circuit.The permanent magnets 34 are disposed at two pole-pitches in thecircumferential direction of the inductor 3C. Thus, the inductor 3C hasmagnetically conductive portions and magnetically non-conductiveportions. The rotary shaft 24 is connected to an engine 20 directly.

[0062] When the rotary shaft 24 is driven by an engine, the inductor 3Cis rotated by the shaft 24 via the retainer plate 35. When field currentis supplied to the field coil 4 by the field current control unit 13,the field coil 4 generates magnetic flux flowing from the field coil 4through the yoke 22, the stator core 21, the inductor 3C to the fieldcoil 4. When the inductor 3C rotates, magnetic flux flowing through theinductor 3C changes because of the magnetically conductive portions andthe magnetically non-conductive portions of the inductor 3C.Accordingly, ac voltage is induced in the armature winding 23. Becausethe permanent magnets 34 are disposed at two pole pitches and magnetizedto have a polarity opposite the direction of the magnetomotive force ofthe stator core 21, the amplitude of change in the magnetic flux thatcrosses the armature winding is increased by the permanent magnets 34.The ac voltage is converted into dc voltage by the inverter unit 17.Because the inductor 3C does not include a cylindrical field coil orclaw poles, the moment of inertia thereof is very small as compared tothe rotor having a Lundell type pole cores. When the motor-generator isoperated as a motor, the inductor can rotates in a very short time whenarmature current is supplied by the inverter 17 because of the smallmoment of inertia of the inductor 3C and series connection of thearmature winding 23 and the field coil 4.

[0063] An ac generator for a vehicle according to the sixty embodimentof the invention will be described hereafter with reference to FIG. 14.

[0064] The ac generator includes a cylindrical front field coil 4 a, acylindrical rear field coil 4 b, a front frame 5 made of magneticmaterial, a rear frame 6 made of magnetic material, a front bearing 8, arear bearing 9, a cylindrical stator core 21 in which a stator winding23 is mounted, a cylindrical inductor 3C made of laminated iron sheetsdisposed inside the stator core 21, a non-magnetic retainer plate 35, arotary shaft 33, and a plurality of permanent magnets 34circumferentially disposed inside the inductor 3C. The non-magneticretainer plate 35 is disposed at the axial middle of the inductor 3C tofix the inductor 3C and the rotary shaft 33 together. The front frame 5and the rear frame 6 jointly hold the stator core 21. The inductor 3Cand the shaft 33 are rotatably supported by the front and rear bearings8, 9. The front frame 5 and the rear frame 6 respectively have acylindrical core portion that axially projects into the inside of theinductor 3C. Each of the cylindrical core portions has an inner borethrough which the rotary shaft 33 extends so as to freely rotate and anend portion having a smaller outside diameter and a base portion havinga larger outside diameter. The inductor 3C is disposed around the endportions of the front and rear frames 1, 7, and the retainer plate 35 isdisposed between the two end portions. The field coil 4 a or 10 b arerespectively disposed around the base portions.

[0065] Other portions are substantially the same as those of the acgenerator according to the fourth embodiment.

[0066] Thus, the outside diameter of the inductor 3C can be made verysmall as compared by a rotor having a Lundell type pole cores, so thatthe moment of inertia thereof can be made very small.

[0067] In the foregoing description of the present invention, theinvention has been disclosed with reference to specific embodimentsthereof. It will, however, be evident that various modifications andchanges may be made to the specific embodiments of the present inventionwithout departing from the scope of the invention as set forth in theappended claims. Accordingly, the description of the present inventionis to be regarded in an illustrative, rather than a restrictive, sense.

What is claimed is:
 1. A rotary electric machine comprising: an armaturehaving an armature core and an armature winding mounted in said armaturecore; a rotor having a rotor core disposed opposite said armature coreand a first magnetic-flux source for supplying first magnetic flux tosaid rotor core, thereby supplying an alternating magnetic field to saidarmature core when said rotor rotates; a frame for supporting saidarmature and said rotor; and a second magnetic flux source, fixed tosaid frame, for supplying second magnetic flux to said rotor core in adirection to supplement said first magnetic flux.
 2. The rotary electricmachine as claimed in claim 1, wherein said second magnetic flux sourcecomprises a yoke for magnetically connecting said armature core and saidrotor core and a stationary field coil for providing dc magnetomotiveforce in a direction opposite the polarity of said rotor core
 3. Therotary electric machine as claimed in claim 1, wherein one of said firstand second magnetic flux sources comprises a permanent magnet, and theother comprises a field coil and means for controlling field currentsupplied to said field coil, and wherein said means controls the fieldcurrent to change magnetic flux supplied by said permanent magnet tosaid armature core.
 4. A rotary electric machine comprising: a statorcore having a three-phase armature winding and a field coil; an inductorrotor disposed opposite said stator core via a first air gap; and amagnetic circuit means for connecting said rotor, said stator core via asecond air gap, wherein said inductor rotor comprises a plurality ofmagnetically conductive portions and magnetically non-conductiveportions that are alternately disposed in the circumferential directionthereof between said first air gap and said second air gap.
 5. Therotary electric machine as claimed in claim 4, wherein said inductorrotor comprises a plurality of permanent magnets having the samepolarity disposed in the circumferential direction thereof at twomagnetic pole-pitches.
 6. The rotary electric machine as claimed inclaim 4, wherein said field coil is disposed inside said inductor rotor.7. The rotary electric machine as claimed in claim 4, further comprisinga rectifier unit connected to said armature winding for providing dcoutput power and a field current control unit connected to said fieldcoil and said rectifier unit for supplying field current to said fieldcoil.